In many surgical procedures the heart is externally bypassed, totally or in part. In a typical procedure, venous blood is removed from the venae cavae and transported through extracorporal equipment which includes generally a pump and an oxygenator. Additionally, there may be included filters and other auxiliary equipment.
The best known pumps employed for the above purpose are the roller pump, the sack-type pump, the diaphragm pump and the piston pump. Typically, pulsatile flow is considered to be superior to non-pulsatile flow for providing the extracorporal transport of blood because it is better for perfusing the rest of the body. It is desirable to provide a flux of blood which corresponds to the rate of movement of the blood within the body of the patient.
The extracorporal movement of blood and the return of that blood to a patient has proven to be particularly difficult due to the composition of blood. Blood consists basically of a particulate-type substance and a liquid substance. The particulate-type substance are red cells or erythrocytes and the liquid is plasma. Plasma comprises, among other components, various gases, crystalloids, electrolytes and protein. The proteins are largely present in the blood as a colloidal suspension which includes platelets. The particulate red cells are suspended in the liquid plasma to form a fluid which is typically termed blood. The red cells constitute about 40% of the fluid volume of blood and normally consist of bi-concave disks about 8 microns in diameter. The red cells contain, among other components, hemoglobin, a chemical carrier for oxygen, carbon dioxide, crystalloids and electrolytes.
A blood pump for the extracorporal movement of blood must avoid damaging the blood. The walls of the blood cells are as fragile as they are important to maintaining the body's adequate metabolism. An acceptable blood pumping device should not bruise or abrade or otherwise damage the blood cells.
Generally, blood can be mechanically insulted during many phases of the pumping process. Mechanical insult to the blood can cause hemolysis or the lysis of red blood cells. Hemolysis is the rupture of the red blood cells with the liberation of their contents, specifically hemoglobin and potassium, into the plasma. Hemolysis causes adverse physiological effects upon the organs perfused with the insulted blood. The mechanical action of a pump acting on the red blood cells can reduce the colloid osmotic pressure of the red blood cells. A reduction in the colloid osmotic pressure disrupts the water retaining properties of the blood. Typically a reduction in the colloidal osmotic pressure results in edema which increases the overall fluid retention and causes a gross swelling of the patient. Denaturation of proteins, specifically platelets, can reduce the osmotic pressure of the plasma. Also, exposure of the blood to mechanical surfaces may cause coagulation of platelets and/or red blood cells to form platelet aggregates at the mechanical surface. These aggregates also reduce osmotic pressure and contribute to edema.
The coagulation also forms microthrombi or larger emboli which can lodge in arteries of smaller diameter. The lodged microthrombi decrease the blood flow and can result in varying degrees of tissue damage. Attempts to circumvent the blockage of arteries by adding an anticoagulant, such as heparin, inhibits healing at the wound surface. Red blood cells can be bruised by contact with mechanical components or by high shear stress. The bruising of red blood cells causes the cells to leak potassium ions. Potassium ions can be toxic if added to plasma in sufficient quantities. Also, white blood cells can be destroyed by mechanical insult. The destruction of white blood cells reduces a patient's ability to resist infection, and can cause numerous complicating factors.
Excessive blood transfusion is not an acceptable solution to the problem of hemolysis, edema, artery blockage, etc. The amount of blood available for transfusion is finite. A blood bank can supply only limited quantities of blood. Numerous problems and limitations exist for matching blood between critically ill patients. Also, excessive blood transfusion tends to greatly reduce the resistance of the patient to infection.
By minimizing all forms of insult to the blood, additional operating time is available for surgery. In fact, mechanical injury to blood by a pumping device almost exclusively determines the time available for surgery to the heart or to the major blood vessels in operations requiring heart bypass. For example, thoracic surgeons give various estimates for the period after which further time on by-pass seriously reduces the chances of the patient surviving. Typically, times of the order of 11/2 to 2 hours are quoted. Some surgeons will even estimate the chances of survival to be as low as 50%, if there is significantly more time on by-pass using typical pumping devices.
A major problem in open-heart surgery is the inability of current technology to provide extracorporal circulation of blood without inducing hemolysis or entraining or forming gas bubbles in the blood. Typically, a pump-oxygenator system is used to provide extracorporal circulation. It is not unusual for blood circulated extracorporally using a pump-oxygenator system to be pumped to the brain of the patient without filtration or a bubble trap. Changes in personality or other detectable changes in mentation have been related to the entrainment of gaseous bubbles in the blood sent to the brain.
Hemolysis, though a problem, does not appear to significantly contribute to changes in personality or other detectable changes in mentation as experienced due to the entrainment of gaseous bubbles in the blood. Typically, extracorporal perfusion is used for periods of two to four hours during a by-pass operation with significant survival periods for patients. However, the problem of bubbles or microbubbles being entrained in the blood and returned to the patient to cause detectable changes in mentation or personality is still a critical problem.
Pump-oxygenator systems produce gaseous bubbles in the blood. Bubbles have been detected in blood from pump-oxygenator systems even when the oxygen and blood are separated by membranes. Considerable effort has been expended in producing filters for removing bubbles from the blood. Such filters have not always proven to be effective. It is believed that bubbles cannot be filtered in the same manner as solid particles, since bubbles, upon impingement on a filter, appear to deform and pass through the filter.
In addition to subtle damage to the brain characterized by changes in mentation and in personality, the entrainment of gaseous bubbles in the blood, the degradation of proteins in the plasma and platelet aggregation can cause other effects such as edema or excessive fluid problems. It is believed that an abnormally excessive accumulation of serous fluid may be collected in connective tissue due to these problems.
There is thus a need for a blood pump apparatus and method adaptable to numerous and varied medical uses, which, provides superior blood flow characteristics compatible with those in the human body, which provides minimal hemolysis and minimal microthrombosis, which reduces the formation and the entrainment of gaseous bubbles in the blood and which minimizes platelet denaturation and aggregation.
An additional feature of the present invention is to provide a blood pump apparatus and method for pressure treating blood.
Another feature of the present invention is to provide a blood pump apparatus and method which dissolves or otherwise disposes of gaseous bubbles during pumping.
A feature of a preferred form of the invention is to provide a blood pump apparatus and method which avoids the sudden interruption of the moving blood which can cause extreme negative pressures, damage to the blood and formation of bubbles.
Yet another feature of the present invention is that the blood itself is not exposed to valve closures.
An additional feature of the present invention is to provide a blood pump apparatus and method which may increase the operating time available to the surgeon without increasing the adverse physiological consequences to the patient.
Another feature of the present invention is to provide a blood pump apparatus and method that simulates the pressure and flow characteristics created by a natural heart.
Additional features and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent from the description, or may be learned by practice of the invention. The objects, features and advantages of the invention may be realized and obtained by means of the instrumentality, combinations and steps particularly pointed out in the appended claims.